Bacteriophage phi29 is a small, well-characterized dsDNA virus that infects Bacillus subtilis. The anti-receptor of phi29 consists of oligomers of the 854-residue protein gp12 and plays an essential role in infection initiation by binding to the receptor on the host cell surface. Oligomers of gp12 exhibit a narrow spindle-shaped configuration 15 nm in length as revealed by electron microscopy and thus are potentially useful nanoscale tools, building blocks, or motor arms. To understand the mechanism of viral infection initiation and to provide a basis for engineering recombinant gp12 for nanotechnology applications, we have initiated structural and bioinformatics studies of gp12. We report here the growth of crystals of gp12 that diffract to 3.0 A resolution. The space group is P3(1)21 or P3(2)21 with unit cell lengths of a = 84.4 A and c = 167.6 A. The asymmetric unit is predicted to contain one gp12 molecule and 32% solvent (VM = 1.8 A3/Da). Domain boundary analysis revealed that gp12 may harbor three domains besides a 24 residue auto-cleave region. The N-terminal half of gp12 contains a domain with about 400 residues that held 44% sequence identity to endopolygalacturonase, a fungal glycosyl hydrolase that catalyzes hydrolysis of the polygalacturonic acid alpha1-4 glycosidic linkage found in plant cell walls. Interestingly, the cell wall of Bacillus subtilis contains a polysaccharide component made from two sugar monomers, N-acetylmuramic acid and N-acetylglucosamine, which resemble alpha-galacturonic acid in that they possess a six-membered pyranose ring. Hence, polygalacturonic acid of plant cell walls and peptidoglycan of bacterial cell walls may offer a similar topography in relation to the polysaccharides. These results suggest a function for gp12 as a cell-wall degrading enzyme in addition to its role in recognition of the host receptor.